The present invention relates to a process for oxidation of alcohol. According to the process of the invention, various organic compounds such as aldehyde compound, ketone compound, lactone compound or the like can be prepared by oxidation of alcohol with high efficiency in a high yield.
In general, the oxidation reaction of alcohol compound is widely utilized in the field of organic synthesis so that numerous processes have been developed.
Especially oxidation reactions using an oxidative catalyst are various and are conducted as useful methods for conversion of functional group which are utilized for numerous types of organic synthesis. Among them, methods using N-oxyl compound as an oxidative catalyst are excellent for selective conversion of functional group in an alcohol and are used in various kinds of oxidation reaction of alcohols.
Conventionally oxidation reactions of alcohols using N-oxyl catalyst have been conducted in an organic solvent or a water-containing organic solvent (Journal by Organic Synthesis Association, published in 1993, vol. 51, pp. 48 to 58, J. Organic Chemistry, 1989, 54, 2970 to 2972, Tetrahedron Letters, 1990, 31, 2177 to 2180, etc.). However, these methods essentially use large amounts of harmful organic solvents such as methylene chloride, so that the methods are undesirable from the viewpoint of preservation of the environment, and are unable to give the contemplated product in fully satisfactory yields.
Also known are methods comprising oxidizing an alcohol in a mixture of water and organic solvent in the presence of N-oxyl compound using hypohalogenous acid salt such as hypochlorite, hypobromite or the like (JP-A-25078/1993, JP-A-211827/1994, etc.). However, the methods also essentially employ an organic solvent and remain to be radically improved for preservation of the environment. Further, when these methods employ sodium hypochlorite and like hypohalogenous acid salt which are low in solubility in an organic solvent, it is difficult to bring about oxidation reaction depending on the kind of alcohol used as the raw material, and the desired product may be produced in a reduced yield. An example of this defect is illustrated in Reference Example 1.
An electrolytic oxidation reaction is electrochemically performed and therefore has drawn attention as a clean oxidation reaction so that some of such reactions are carried out on an industrial scale.
Generally conventional electrolytic alcohol oxidation reaction methods include a direct electrolytic oxidation method to be effected in an organic solvent or a water-containing organic solvent as described in “Electrolytic Organic Synthesis” (Sigeru TORII, published by Kodansha Publishing Co., 1981, pp. 262 to 273), an indirect electrolytic oxidation method using a mediator (electron carrier), and electrolytic oxidation method involving the use of 2-layer system (2 layers each containing water and a water-immiscible organic solvent, respectively) (J. Org. Chem., 1991, 56, 2416 to 2421).
Many reaction systems of these electrolytic oxidation methods use organic solvents which entail difficulty in flow of electric current so that the methods require a large amount of supporting electrolyte, e.g. 10 to 50 wt. % of supporting electrolyte when using N,N-dimethylformamide as a solvent. The large amount of supporting electrolyte used raises a problem of necessitating a cumbersome procedure for isolating and purifying the obtained reaction product as well as problems of high costs and disposal of waste. Also known are methods using various metal catalysts as the mediator. Among them, commercially feasible methods are limited because of serious problems such as high costs of metal compounds and post-treatment.
N-oxyl compound is reportedly superior as a catalyst for electrolytic oxidation reaction of alcohol compound (J. Org. Chem., 1991, 56, 2416 to 2421). Any reactions disclosed in this document, however, are of two-layer system type (methylene chloride/water) and can not be used these days because it is difficult to industrially use methylene chloride for the environmental problem.
As described above, the oxidation reaction of alcohol compound still entails various problems to be overcome or mitigated. Therefore there is a need for development of oxidation reaction which is industrially practical.
An object of the present invention is to provide a novel process for oxidizing an alcohol, the process being capable of giving the contemplated product in a high yield by a simplified reaction operation irrespectively of the type of alcohol used as the raw material without a need to use an organic solvent which is likely to adversely affect the environment, the process being applicable to all kinds of industrial manufacture.
Another object of the invention is to provide a general-purpose process for oxidizing an alcohol, the process comprising subjecting an alcohol compound which is sparingly soluble in water to an electrolytic oxidation in water, the process being free from the drawbacks of conventional electrolytic oxidation processes and capable of producing an oxide which is higher in oxidizing degree than the alcohol as the raw material with high efficiency in a high yield.